Electro-optic devices have shown great promise in sensor applications. Passive optical sensors offer safe and accurate operation in hostile environments of heat and temperature, while also being immune to electromagnetic interference. These advantages combine to make optically-based sensors attractive for a number of applications.
A resonant optical sensor is generally defined as a sensor based on optical resonators such as, for example, Fabry-Perot devices, spheres, rings, disks and the like, which are used as physical, chemical and/or biological sensors of the ambient medium. In most cases, the spectrum of the sensor consists of positive and/or negative resonant peaks. The sensor monitors changes in the tested “object” (e.g., ambient medium) by measuring and processing variations in the spectrum. U.S. Pat. No. 6,901,101 issued to R. L. Frick on May 31, 2005 is exemplary of this type of resonant optical sensor and includes an optical medium having a ‘variable’ cavity; that is, the cavity is physically altered in response to changes in a measurable parameter such as pressure, temperature, force, flow rate, etc. The cavity is disposed near or within a high Q optical resonator, where the variability of the cavity affects the resonance condition within the high Q resonator.
It is often necessary in arrangements such as disclosed by Frick to detect very small variations in the spectrum, comparable with or less than the dimensions of a resonant peak itself. Commonly, it is assumed that the higher the Q-factor of the resonator, the more sensitive the resonator is to changes in the refractive index (indeed, a very large Q-factor denotes an extremely narrow resonance in the transmission power spectrum). However, the maximum Q-factor of an individual resonance peak corresponds to a resonance peak with vanishing height, which is useless for sensing. Moreover, a resonance peak composed of several adjacent peaks may have a complex shape, for which the Q-factor cannot be easily determined.
Thus, a need remains for ascertaining a proper set of parameters that may be used to optimize the sensitivity of a resonant optical sensor.